Ultrasonics Sonochemistry 18 (2011) 917–922

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Ultrasonics Sonochemistry

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Short Communication

Ultrasound-assisted synthesis of 2,5-dimethyl-N-substituted pyrroles catalyzedby uranyl nitrate hexahydrate

V.S.V. Satyanarayana, A. Sivakumar ⇑Chemistry Division, School of Advanced Sciences, VIT University, Vellore 632 014, India

a r t i c l e i n f o

Article history:Received 15 December 2010Received in revised form 4 February 2011Accepted 6 February 2011Available online 27 February 2011

Keywords:2,5-Dimethyl-N-substituted pyrrolesUO2(NO3)2.6H2OUltrasonic irradiationSoft conditions

1350-4177/$ - see front matter � 2011 Elsevier B.V. Adoi:10.1016/j.ultsonch.2011.02.007

⇑ Corresponding author.E-mail address: profaskumar09@gmail.com (A. Siv

a b s t r a c t

An efficient synthesis of different novel 2,5-dimethyl-N-substituted pyrrole derivatives by the Paal–Knorrcondensation has been accomplished using uranyl nitrate hexahydrate as catalyst under soft conditionsand ultrasonic irradiation. The synthesized compounds were confirmed through spectral characterizationusing IR, 1H NMR, 13C NMR and mass spectra.

� 2011 Elsevier B.V. All rights reserved.

1. Introduction

The pyrrole scaffold is an useful structural pattern for exhibitingchemical functionality in biologically active molecules [1]. It hasestablished broad application in drug development for the treat-ment as antibacterial, anti-inflammatory, antiviral, antitumoral,and antioxidant agent [2]. The pyrrole ring system is one of themost important substructures for biologically active compoundssuch as indolizidine alkaloids, unsaturated g-lactams and bicycliclactams [3]. These structural units are found in a wide array of nat-ural products, synthetic materials and bioactive molecules such asvitamin B12, heme and cytochromes [1,4]. Therefore, preparationof pyrroles has attracted considerable attention of chemists in re-cent years. There are several methods for the synthesis of pyrrolein the literature [5] from classical Hantzsch procedure [6], 1,3-dipo-lar cycloaddition reaction [7], aza-Wittig reaction [8], reductivecoupling [9], titanium catalyzed hydroamination of diynes [10]and other multistep operations [11]. The most widely used methodis the Paal–Knorr synthesis, which involves the cyclocondensationreaction of 1,4-dicarbonyl compounds with primary amines to pro-duce substituted pyrroles. Several catalysts have been used in thisreaction including HCl [12], p-TSA [13], HOAc [14], H2SO4 [15], I2

[16], different metal complexes [17–30], and montmorillonite[16,31–32]. In addition, the above cyclocondensation process couldproceed in ionic liquid [33], microwave irradiation [34] or ultra-sound irradiation using ZrCl4 as catalyst [35]. However, some ofthe synthetic protocols for the synthesis of 2,5-dimethyl-N-substi-

ll rights reserved.

akumar).

tuted pyrroles reported above suffer from one or more disadvan-tages, such as harsh reaction conditions, poor yields, prolongedreaction time period as well as use of corrosive, expensive reagents,limiting these methods. Thus, simple, efficient and flexible proto-cols for the synthesis of 2,5-dimethyl-N-substituted pyrroles arestill the need as there is scope for further improvement towardsmilder reaction conditions, development of simple and inexpensivereagents convenient procedures and higher product yields.

Ultrasonic-assisted organic synthesis as a green synthetic ap-proach is a powerful technique that is being used more and moreto accelerate organic reactions [36,37]. A large number of organicreactions can be carried out in higher yields, shorter reaction timeor milder conditions under ultrasound irradiation and considered aprocessing aid in terms of energy conservation and waste minimi-zation which compared with traditional methods.

Uranyl nitrate is usually used as an electron dense stain fortransmission electron microscopy. However, it has also foundapplication as 0.01% aqueous solution as a local catalyst in thepolymerization of methacrylates [38].

We report herein, a simple, mild and expeditious synthesis of2,5-dimethyl-N-substituted pyrroles in high yields by reacting1,4-dicarbonyl compound with substituted primary amines usinguranyl nitrate hexahydrate [UO2(NO3)2.6H2O] (Scheme 1) as cata-lyst under soft conditions and ultrasonic irradiation.

2. Results and discussion

2,5-Dimethyl-N-substituted pyrroles (3a–k) were synthesizedby the cyclocondensation of commercially available 2,5-hexanedi-one with different substituted primary amines using uranyl nitrate

R-NH2 +

O

O

N

R

UO2(NO3)2.6H2O

MeOH, r.t. / ))))1 2a 3

Scheme 1. Synthesis of 2,5-dimethyl-N-substituted pyrroles catalyzed by uranyl nitrate hexahydrate.

Table 2Synthesis of 2,5-dimethyl-N-phenyl pyrrolea (3a) using 10 mol% of UO2(NO3)2.6H2Oin different solvent systems.

Entry Solvent Time (min) Yieldb (%)

1 Acetonitrile 120 792 CH2Cl2 300 573 CHCl3 300 614 Ethanol 60 885 Methanol 30/5c 94/96c

6 Solvent free 60 83

a Aniline: 2,5-Hexanedione (1 mmol: 1.1 mmol).b Isolated yield.c At room temperature/Ultrasonic irradiation.

918 V.S.V. Satyanarayana, A. Sivakumar / Ultrasonics Sonochemistry 18 (2011) 917–922

hexahydrate [UO2(NO3)2.6H2O] (Scheme 1) as catalyst under softconditions and ultrasonic irradiation. To optimize the reaction con-ditions, the reaction between 2,5-hexanedione and aniline wasused as a model reaction. The results shown in Table 1 clearly indi-cate the scope and generality of the reaction with respect to vari-ous catalytic systems. In comparison with other catalysts such asBi(NO3)2.5H2O, [BMIm]I and RuCl3 which have been recently re-ported in the synthesis of 2,5-dimethyl-N-substituted pyrroles,UO2(NO3)2.6H2O employed here shows a more effective catalyticactivity than the others in terms of amount of catalyst, yield andreaction time (Table 1, entry 8–10).The influence of other catalystssuch as Sr(NO3)2, Mg(NO3)2.6H2O were also examined. It is notablethat strontium nitrate was found to give quite high yields for thistransformation compared with magnesium nitrate hexahydrate(Table 1, entry 6 & 7). However, in the absence of catalyst, the reac-tion yielded only 22% of product after 24 h of reaction.

Before taking up the reaction using ultrasonic irradiation, it wastried out using different solvents such as methanol, ethanol, aceto-nitrile, dichloromethane and chloroform as well as solvent freesystem under normal reaction conditions with 10 mol% of UO2-(NO3)2.6H2O as the catalyst (Table 2). However, it was noticed thatthe highest yield and shorter reaction duration was achievablewith methanol while the others took longer duration to yield lesserproduct. Hence the ultrasonic irradiation which required only5 min duration to complete reaction of aniline with 2,5-hexanedi-one and the product yield was 96% is compared to the normal reac-tion conditions, stirring at room temperature which have taken30 min for reaction to completion and yield was slightly less(94%) with methanol as solvent (Table 2, entry 5). It was clear thatthe ultrasound could accelerate the reaction of amine and 2,5-hexancedione.

The following is expected to be plausible reason for the higheryield and lesser reaction time during ultrasonic irradiation:

Cavitation is a process of formation of bubbles having dynamiclife during ultrasonic irradiation. These bubbles can be filled withgas or vapour and occur in organic solvents used in the ultrasonicirradiation. When these bubbles burst, it results in high tempera-ture and high pressure which facilitate the intermolecular reaction.

Table 1Comparison of effect of catalysts in the formation of 2,5-dimethyl-N-phenyl pyrrolea

(3a) at room temperature.

Entry Catalyst Mole % Time (h) Yieldb (%)

1 No Catalyst – 24 222 UO2(NO3)2.6H2O 1 2 573 UO2(NO3)2.6H2O 2.5 2 694 UO2(NO3)2.6H2O 5 1 885 UO2(NO3)2.6H2O 10 0.5/5c 94/96c

6 Sr(NO3)2 20 4 737 Mg(NO3)2.6H2O 20 4 558 Bi(NO3)2.5H2O 100 10 96[19]9 [BMIm]I 1.5 g 3 96[33]10 RuCl3 5 0.5 94[30]

a Aniline: 2,5-Hexanedione (1 mmol: 1.1 mmol).b Isolated yield.c At room temperature (h)/ultrasonic irradiation (min).

Apart from this, the shock wave produced by the bubble collapsecan disrupt the solvent structure which can influence the reactivityby altering solvation of the reactive species present in the reactionmixture. Sonochemical rate enhancement is a known phenomenonin organic reactions [39]. It can be attributed that by the samemechanism, the yield of 2,5-dimethyl-N-substituted pyrroles ishigher in ultrasound than in normal chemical reaction.

In order to evaluate the generality of the process, several diver-sified examples illustrating the present method for the synthesis of2,5-dimethyl-N-substituted pyrroles (3a–k) was studied (Table 3).The reaction of 2,5-hexanedione (2a) with various aromatic/ali-phatic substituted primary amines bearing electron donatinggroups, electron withdrawing groups and several heterocyclic moi-eties was carried out in the presence of UO2(NO3)2.6H2O as catalystunder soft conditions and ultrasonic irradiation. The yields ob-tained were good to excellent without formation of any side prod-ucts and these results are illustrated in Table 3. From theseexperiments, it is clearly demonstrated that the uranyl nitratehexahydrate [UO2(NO3)2.6H2O] is indeed an effective catalyst andis undoubtedly superior to other catalysts, procedures with respectto reaction time, availability of catalyst, work-up procedure andyields. The compounds IVa–e were synthesized by refluxing differ-ent substituted acetohydrazides (IIIa–e) and 2,5-hexanedione inthe presence of UO2(NO3)2.6H2O. The reaction time and yields ofthe compounds IVa–e are depicted in Table 4. The compoundsIIIa–e were synthesized by refluxing the mixture of ethyl(substi-tuted)mono/diacetate (IIa–e) and hydrazine hydrate in ethanol.The compounds IIa–e were synthesized by the reaction of ethylchloroacetate with different mono and dihydroxy substituted aro-matic compounds in the presence of K2CO3 as catalyst (Scheme 2)[40]. Structure of new derivatives of 2,5-dimethyl-N-substitutedpyrroles (3b–c, 3k and IVa–e) properly characterized by their IR,1H NMR, 13C NMR and mass spectra, while the known compoundswere identified by comparison of their spectroscopic data andmelting points with the reported values in literature.

3. Experimental

3.1. General

All reagents purchased from Aldrich, Hi Media and Qualigenswere used without further purification. UO2(NO3)2.6H2O

QX H + ClO

O

K2C

Stirr

O

CaRe

QX

HN

O

N

X = O, NQ = aryl / heteroaryl

substituents

(I)

(IV)

Scheme 2. Synthetic protoc

Table 3Synthesis of 2,5-dimethyl-N-substituted pyrroles (3a-k) catalyzed by uranyl nitratehexahydrate.

Entry Amine Product Time(min)a

Yield(%)b

M.P.(�C)

1 Ph-NH2

Ph N

3a

30/5 94/96 48–50

2NH2Br

NBr

3b

60/6 89/90 56–58

3NH2I

NI

3c

60/6 86/88 62–64

4NH2H3CO

NH3CO

3d

45/6 86/84 60–62

5NH2O2N

NO2N

3e

60/5 87/90 144–146

6N NH2

N

N N NN

N3f

120/8 76/72 140–142

7 NH2

N

3g

120/10 83/81 120–122

8 NH2(CH2)2NH2H2C-CH2

N N

3h

180/15 87/85 134–136

9H2N NH2

N N

3i

180/15 90/87 248–250

10 PhCH2NH2

PhH2C N3j

90/6 88/86 40–42

11CONHNH2

CONHN

3k

300/16 81/83 156–158

a Under reflux conditions/Ultrasonic irradiation.b Isolated yield.

V.S.V. Satyanarayana, A. Sivakumar / Ultrasonics Sonochemistry 18 (2011) 917–922 919

purchased from LOBA Chemie and used as received. Infrared (IR)spectra were recorded at room temperature from 4000 cm�1 to400 cm�1 with KBr pellets at a resolution of 4 cm�1, using Avatar330 equipped with DTGS detector. Most of the obtained vibrationalbands of the IR spectrum were identified and compared with thosevalues available in literature. The 1H NMR was recorded on aBruker AMX-400 (400, 500 MHz) instrument at room temperatureusing the X-WIN NMR version X-WIN NMR 1.3 cn drx software.Approximately 0.03 M solutions in CDCl3 or DMSO-d6 using TMSas internal reference were used for recording 1H NMR spectra.The accuracy of the 1H shifts considered as 0.02 ppm. The couplingconstants J are in Hz. 13C NMR spectra were recorded on BrukerAvance III (125.75 MHz) spectrometer. Mass spectra were obtainedusing JEOL GC MATE II HRMS (EI) mass spectrometry. Sonicationwas performed in a SONICS, Vibra Cell, VC 130, ultrasonic proces-sor equipped with a 3 mm wide and 140 mm long probe, whichwas immersed directly into the reaction mixture. The operatingfrequency was 20 kHz and the output power was 0–130 Wattthrough manual adjustment. Melting points were determined inopen capillaries and are uncorrected.

3.2. Procedure for the synthesis of 2,5-dimethyl-N-substituted pyrroles(3a–k)

3.2.1. Method ATo a solution of the amine 1 (1 mmol) and 2,5-hexanedione 2a

(1.1 mmol) in methanol (2 mL) at room temperature, uranyl nitratehexahydrate (10 mol%) was added. The mixture was stirred atroom temperature for a period specified in Table 3. The progressof reaction was monitored by TLC. After completion of reactionthe resulting mixture was filtered, washed with a 5% HCl solutionto remove the excess amine and washed with a small amount ofcold water. The crude product was weighed and recrystallizedfrom 10% aqueous methanol. The reaction mixture was cooled inice and the product was collected and dried.

3.2.2. Method BTo a solution of the amine 1 (1 mmol) and 2,5-hexanedione 2a

(1.1 mmol) in methanol (2 mL) at room temperature, uranyl nitratehexahydrate (10 mol%) was added and the resulting reaction mix-ture was subjected to ultrasonic irradiation using ultrasonic pro-cessor equipped with a 3 mm wide and 140 mm long probe,which was immersed directly into the reaction mixture for a periodas shown in Table 3. The operating frequency was 16 kHz and theoutput power was 104 Watt through manual adjustment. Theprogress of reaction was monitored by TLC. After completion of

O3, DMF

at 80 oCQX

O

O

H2N-NH2.H2O

QX

HN

O

NH2O

talyst,flux / ))))

(II)

(III)

ol of compounds IVa–e.

Table 4Synthesis of compounds (IVa–e) using uranyl nitrate hexahydrate as catalyst.

Entry Reactant Product Time (h/min)a Yield (%)a,b M.P (�C)

1

ONH

O

NH2

(IIIa) (IVa)

ONH

O

N

2/16 87/84 204–206

2

(IIIb)

O ONH

ONH2O

(IVb)

O ONH

O

NO

4/20 79/73 214–216

3

(IIIc)

HN

O

NH2N

N

Ph

PhPh (IVc)

HN

O

NN

N

Ph

PhPh

4/20 82/75 240–242

4

(IIId)O O

HN

O

HN

ONH2H2N

(IVd)O O

HN

O

HN

ONN

6/25 76/71 224–226

5

(IIIe)O OHN

O

HN

ONH2H2N (IVe)O O

HN

O

HN

ONN

6/25 71/68 170–172

a Under reflux conditions/Ultrasonic irradiation.b Isolated yield.

920 V.S.V. Satyanarayana, A. Sivakumar / Ultrasonics Sonochemistry 18 (2011) 917–922

reaction the resulting mixture was filtered, washed with a 5% HClsolution to remove the excess amine and washed with a smallamount of cold water. The separated product was filtered, washedwith ice cold water, recrystallized from 10% aqueous methanol anddried at room temperature.

3.3. 1-(4-phenyl)-2,5-dimethyl-1H-pyrrole (3a)

mp 48–50 �C. IR (KBr, mmax, cm�1): 3053, 2922, 1596, 1491,1400, 1316, 1218, 1037, 999, 747. 1H NMR (500 MHz, CDCl3) dH:7.49 (m, 2H, m,m’-ArH), 7.43 (m, 1H, p-ArH), 7.25 (m, 2H, o,o’-ArH) 5.94 (s, 2H, pyrrole), 2.07 (s, 6H, 2CH3). 13C NMR(125.75 MHz, CDCl3) dC: 139.02, 129.05, 128.82, 128.27, 127.63,105.63, 13.02. HRMS (EI): m/z [M]+ calcd for C12H13N: 171.10;found: 171.1059.

3.4. 1-(4-Bromophenyl)-2,5-dimethyl-1H-pyrrole (3b)

mp 56–58 �C. IR (KBr, mmax, cm�1): 3078, 2917, 1519, 1484,1435, 1402, 1320, 1065, 998, 840, 760. 1H NMR (300 MHz, CDCl3)dH: 2.01 (s, 6H, 2CH3), 5.79 (s, 2H, pyrrole), 7.08 (d, 2H, J = 8.4 Hz,o,o0-ArH), 7.57 (d, 2H, J = 8.4 Hz, m,m’-ArH). 13C NMR(125.75 MHz, CDCl3) dC: 12.97, 106.13, 121.53, 128.67, 129.88,132.32, 138.07. HRMS (EI): m/z [M + H]+ calcd for C12H12BrN:249.0153; found: 250.1000.

3.5. 1-(4-Iodophenyl)-2,5-dimethyl-1H-pyrrole (3c)

mp 62–64 �C. IR (KBr, mmax, cm�1): 3073, 2916, 1583, 1484,1434, 1403, 1321, 1053, 997, 838, 760. 1H NMR (500 MHz, CDCl3)dH: 2.06 (s, 6H, 2CH3), 5.94 (s, 2H, pyrrole), 7.01 (d, 2H, J = 8.5 Hz,o,o’-ArH), 7.83 (d, 2H, J = 8.5 Hz, m,m’-ArH). 13C NMR(125.75 MHz, CDCl3) dC: 13.06, 92.98, 106.18, 128.66, 130.17,138.34, 138.74. HRMS (EI): m/z [M]+ calcd for C12H12IN:297.0014; found: 297.1000.

3.6. N-(2,5-dimethyl-1H-pyrrol-1-yl)benzamide (3k)

mp 156–158 �C. IR (KBr, mmax, cm�1): 3263, 3062, 2934, 1664,1602, 1537, 1488, 1282, 748. 1H NMR (300 MHz, CDCl3) dH: 2.13(s, 6H, CH3 on pyrrole), 5.77 (s, 2H, CH on pyrrole), 7.49 (m, 3H,ArH), 7.86 (d, 2H, J = 7.2 Hz, ArH), 8.15 (s, 1H, CONH). 13C NMR(125.75 MHz, CDCl3) dC: 11.47, 103.61, 127.39, 127.50, 127.96,128.12, 128.71, 128.76, 129.20, 131.51, 131.75, 132.39, 132.78,166.28. ESI-MS: m/z calcd for C13H14N2O: 214.11; found: 214.51[M]+. HRMS (EI): m/z [M]+ calcd for C13H14N2O: 214.1106; found:214.1200.

3.7. Procedure for the synthesis of compounds (IVa–e)

3.7.1. Method ATo a suspensions of different substituted acetohydrazides IIIa–e

(0.001 mol) in methanol/chloroform (1:1) mixture (20 mL) wereadded acetonyl acetone (0.002 mol) and uranyl nitrate hexahy-drate (10 mol%) and the reaction mixture was refluxed for a periodspecified in Table 4. The reaction mixture was concentrated to halfof the original volume and poured into crushed ice. The separatedsolid was filtered, washed with water and dried.

3.7.2. Method BTo a suspensions of different substituted acetohydrazides IIIa–e

(0.001 mol) in methanol/chloroform (1:1) mixture (20 mL) wereadded acetonyl acetone (0.002 mol) and uranyl nitrate hexahy-drate (10 mol%) and the resulting reaction mixture was subjectedto ultrasonic irradiation using ultrasonic processor equipped witha 3 mm wide and 140 mm long probe, which was immersed di-rectly into the reaction mixture for a period as shown in Table 3.The operating frequency was 16 kHz and the output power was104 Watt through manual adjustment. The progress of reactionwas monitored by TLC. After completion of reaction the resultingmixture was poured into crushed ice. The separated product wasfiltered, washed with ice cold water and dried at roomtemperature.

V.S.V. Satyanarayana, A. Sivakumar / Ultrasonics Sonochemistry 18 (2011) 917–922 921

3.8. N-(2,5-dimethyl-1H-pyrrol-1-yl)-2-(naphthalen-2-yloxy)acetamide (IVa)

mp 204–206 �C. IR (KBr, mmax, cm�1): 3216, 3060, 2923, 1688,1629, 1511, 1465, 1432, 1391, 1256, 1219, 1184, 1122, 1068, 963,844, 754. 1H NMR (400 MHz, CDCl3) dH: 1.61 (s, 3H, CH3 on pyrrole),2.02–2.41 (m, 3H, CH3 on pyrrole), 4.60 (s, 1H, OCH), 4.83–4.91 (m,1H, OCH), 6.85 (s, 2H, CH on pyrrole), 7.00–7.14 (m, 4H, ArH), 7.57–7.82 (m, 3H, ArH), 8.26 (s, 1H, CONH). 13C NMR (125.75 MHz, CDCl3)dC: 11.41, 66.59, 103.60, 107.73, 119.20, 124.46, 127.06, 127.14,127.36, 128.04, 129.32, 129.88, 134.41, 155.90, 167.85. ESI-MS: m/zcalcd for C18H18N2O2: 294.14; found: 295.34 [M + H]+. HRMS (EI):m/z [M]+ calcd for C18H18N2O2: 294.1368; found: 294.3500.

3.9. N-(2,5-dimethyl-1H-pyrrol-1-yl)-2-(4-methyl-2-oxo-2H-chromen-7-yloxy)acetamide (IVb)

mp 214–216 �C. IR (KBr, mmax, cm�1): 3258, 3087, 2921, 1716,1687, 1616, 1504, 1427, 1390, 1295, 1202, 1156, 1079, 846, 776.1H NMR (400 MHz, CDCl3) dH: 1.52 (s, 3H, CH3 on coumarin),2.07 (s, 6H, 2CH3 on pyrrole), 4.89 (s, 2H, OCH2), 5.90 (s, 2H, CHon pyrrole), 7.23 (s, 1H, CH on coumarin), 7.42 & 7.49 (2d, 1H,J = 6.8 & 7.6 Hz, ArH), 7.75–7.84 (m, 2H, ArH), 8.62 (s, 1H, CONH).13C NMR (125.75 MHz, CDCl3) dC: 11.39, 18.60, 66.75, 102.27,103.65, 112.06, 112.99, 114.29, 127.03, 127.31, 153.82, 154.95,160.48, 160.92, 167.32. ESI-MS: m/z calcd for C18H18N2O4:326.13; found: 327.36 [M + H]+. HRMS (EI): m/z [M]+ calcd forC18H18N2O4: 326.1267; found: 326.5646.

3.10. N-(2,5-dimethyl-1H-pyrrol-1-yl)-2-(2,4,5-triphenyl-1H-imidazol-1-yl)acetamide (IVc)

mp 240–242 �C. IR (KBr, mmax, cm�1): 3251, 3056, 2923, 1685,1213 cm�1. 1H NMR (500 MHz, DMSO-d6) dH: 1.80 (s, 6H, C2,5–CH3

on pyrrole ring), 4.65 (s, 2H, N-CH2-CO), 5.63 (s, 2H, C3,4–H on pyrrolering), 7.15 & 7.22 (2t, 3H, J = 7.75 Hz, p-ArH of 2,4,5-phenyl rings),7.39–7.44 (m, 4H, o,o0-ArH of 4,5-phenyl rings), 7.52–7.54 (m, 6H,m,m’-ArH of 2,4,5-phenyl rings), 7.66–7.68 (m, 2H, o,o’-ArH of 2-phe-nyl ring), 10.88 (s, 1H, CONH). 13C NMR (125.75 MHz, CDCl3) dC:10.39, 10.75, 45.90, 104.00, 105.24, 126.49, 126.70, 126.83, 127.11,127.33, 127.92, 128.07, 128.12, 128.57, 128.74, 128.96, 129.07,129.15, 129.18, 129.25, 129.42, 129.89, 130.13, 130.21, 130.30,130.66, 130.87, 131.19, 134.04, 137.66, 148.37, 148.48, 171.07. LC-MS: m/z calcd for C29H26N4O: 446.21; found: 447.2 [M + H]+. HRMS(EI): m/z [M]+ calcd for C29H26N4O: 446.2107; found: 446.5400.

3.11. 2,2’-(Naphthalene-2,7-diylbis(oxy))bis(N-(2,5-dimethyl-1H-pyrrol-1-yl)acetamide) (IVd)

mp 224–226 �C. IR (KBr, mmax, cm�1): 3324, 2920, 1707, 1631,1514, 1435, 1389, 1252, 1209, 1162, 1062, 835, 754. 1H NMR(400 MHz, CDCl3) dH: 2.05 (s, 12H, CH3 on pyrrole), 4.84 (s, 4H,OCH2), 5.82 (s, 4H, CH on pyrrole), 7.12 (s, 2H, ArH on C1 & C8 ofnaphthalene ring), 7.13 (d, 2H, J = 8.2 Hz, ArH on C3 & C6 of naphtha-lene ring), 7.78 (d, 2H, J = 8.6 Hz, ArH on C4 & C5 of naphthalene ring),8.74 (s, 2H, CONH). 13C NMR (100.6 MHz, CDCl3) dC: 10.81, 11.15,66.99, 103.95, 104.46, 107.20, 116.23, 125.72, 127.74, 130.04,135.32, 155.68, 167.01. ESI-MS: m/z calcd for C26H28N4O4: 460.21;found: 460.50 [M]+. HRMS (EI): m/z [M]+ calcd for C26H28N4O4:460.2111; found: 460.5200.

3.12. 2,2’-(4,4’-(propane-2,2-diyl)bis(4,1-phenylene)) bis(oxy)bis(N-(2,5-dimethyl-1H-pyrrol-1-yl)acetamide) (IVe)

mp 170–172 �C. IR (KBr, mmax, cm�1): 3237, 2971, 1689, 1506,1435, 1246, 1210, 1182, 1068, 831, 753, 580. 1H NMR (300 MHz,

CDCl3) dH: 1.57–1.65 (m, 6H, CH3 on bisphenol), 2.05 (s, 12H, CH3

on pyrrole), 4.75 (s, 4H, OCH2), 5.80 (s, 2H, CONH), 6.89 (d, 4H,J = 8.4 Hz, ArH), 7.19 (d, 4H, J = 8.4 Hz, ArH), 8.57 (s, 2H, CONH).13C NMR (125.75 MHz, CDCl3) dC: 11.36, 31.19, 41.80, 66.61,103.55, 114.74, 127.34, 127.89, 144.00, 155.79, 168.03. ESI-MS:m/z calcd for C31H36N4O4: 528.27; found: 529.52 [M + H]+. HRMS(EI): m/z [M]+ calcd for C31H36N4O4: 528.2737; found: 528.1000.

4. Conclusion

It may be reasonably concluded that the present procedure forthe synthesis of 2,5-dimethyl-N-substituted pyrroles through aone pot condensation of 1,4-dicarbonyl compounds and substi-tuted primary amines establishes the potential of uranyl nitratehexahydrate [UO2(NO3)2.6H2O] as a good catalyst under soft condi-tions/ultrasonic irradiation for such condensation reactions andgives hope for further useful applications in organic synthesis.Moreover, this methodology offers substantial advantages with re-spect to simplicity of operation, yield of products, reaction times,availability of catalyst, choice of substituents on the pyrrole ring,easy work-up procedure under mild reaction conditions. Pyrroleswith different N-substituted heterocyclic moieties (IVa–e) wereprepared by using uranyl nitrate hexahydrate as catalyst under re-flux conditions and ultrasonic irradiation.

Acknowledgment

Authors, express thanks to the Administration of VIT University,Vellore for providing required facilities to support this work.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.ultsonch.2011.02.007.

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